I was seeing the same problem. If the page reloads while scrolled, the first image in the slider will be correctly positioned but the second and subsequent images will be positioned with their tops aligned with the top edge of the window. Then, when you scroll back to the top of page, the parallax effect adjusts them down and the white background appears.
I think the culprit is the jQuery offset() function returning the wrong value. This seems to be because slider images other than the 1st have had their css display property set to ‘none’ by the superslide script before the parallax script is called for the first time.
My solution was to edit the Parallax function in scripts.js and replace the following section:
with code that temporarily sets the display property to ‘block’ before calling offset() and then reverting it back to its previous value:
(I’ve added indentation to make it easier to read)
Hope this is useful - it seems to have solved the problem for me.